A preferred application of the invention is in “double-phase” motors. This may be explained as follows: because handy abbreviations are always desirable in engineering, and a differentiation in terms of “two-pulse” and “two-phase” is too complex for normal commerce, the generic term “double-phase motors” is usually used for such motors, regardless of whether such a motor has one or two phases.
Because, for example, a two-pulse motor receives two stator current pulses (which in some circumstances can be very short, e.g. only 10° el., and can also be subdivided into even shorter pulses by pulse width modulation) in its stator for each rotor revolution of 360° el., in motors of this design, a gap in the electromagnetically generated torque occurs between the two stator current pulses, and this gap is bridged by an auxiliary torque of arbitrary nature. This can be a mechanically generated auxiliary torque, but in the majority of cases it is a so-called “reluctance torque,” which results from the interaction of a permanent-magnet rotor with the iron masses of the stator.
For this purpose, the iron masses of the stator must be distributed inhomogeneously with respect to the stator's circumference; there are infinite possibilities for this. When the permanent-magnet rotor of a (currentless) motor of this kind is then driven at uniform speed, it induces an alternating voltage in the stator winding, i.e. the motor then works as an alternating-current generator. The induced alternating voltage deviates from the sinusoidal shape, and its shape additionally depends on the rotation direction. These factors must be taken into account in the design of such motors.